DC/DC converter

1. A DC/DC converter comprising: a switching element connected to an input voltage so as to be turned on and off; a driving circuit arranged to perform ON/OFF control of the switching element; an inductor arranged to flow a current controlled by the switching element; a smoothing capacitor connected to the inductor so as to perform rectifying operation together with the inductor; an oscillator arranged to generate a rectangular wave signal for operating the driving circuit; and an output current detecting portion arranged to detect an output detection current flowing in the switching element or the inductor, wherein the oscillator generates the rectangular wave signal at a fixed oscillation frequency when the output detection current is a predetermined value or larger, whereas the oscillator generates the rectangular wave signal at an oscillation frequency lower than the fixed oscillation frequency and proportional to the output detection current when the output detection current is smaller than the predetermined value, wherein the oscillator includes a constant current source arranged to generate a constant current regardless of the output detection current, and a linear current source arranged to generate a linear current responding to the output detection current, and the linear current is fixed to a first current value when the output detection current is smaller than a first threshold value, and the linear current is variably controlled from the first current value to a second current value responding to the output detection current when the output detection current is larger than the first threshold value and smaller than a second threshold value, and the linear current is fixed to the second current value when the output detection current is larger than the second threshold value.

2. The DC/DC converter according to claim 1 , wherein an oscillation frequency of the oscillator is set based on an oscillator current corresponding to the sum of the constant current and the linear current or a difference between them.

3. The DC/DC converter according to claim 1 , wherein a PWM control is performed when the output detection current is a predetermined value or larger, and a PFM control is performed when the output detection current is smaller than the predetermined value.

4. The DC/DC converter according to claim 3 , wherein setting of an oscillation frequency of the oscillator is controlled by the linear current when the PFM control is performed.

5. The DC/DC converter according to claim 3 , wherein a minimum value of the oscillation frequency of the oscillator is above an audible frequency band when the PFM control is performed.

6. The DC/DC converter according to claim 1 , further comprising: an error amplifier arranged to compare a feedback voltage generated based on a voltage generated at the smoothing capacitor with a reference voltage having a predetermined value so as to output a difference between the both voltages as an error signal; a slope voltage generating circuit arranged to generate a slope signal based on a rectangular wave signal generated by the oscillator; and a PWM comparator arranged to output a signal of a comparison result between the error signal and the slope signal.

7. The DC/DC converter according to claim 6 , wherein a voltage component corresponding to the output detection current is superimposed on the slope signal.

8. The DC/DC converter according to claim 1 , further comprising a rectifier element connected to a common node with the switching element and the inductor so as to supply current to the inductor when the switching element is turned off.

9. The DC/DC converter according to claim 8 , further comprising a reverse current detection circuit arranged to detect reverse current flowing from a ground potential to the common node, wherein when the reverse current detection circuit detects a predetermined reverse current, operation of the rectifier element or the switching element connected to the ground potential is turned off.

10. The DC/DC converter according to claim 6 , wherein the error amplifier is a transconductance type.

11. The DC/DC converter according to claim 1 , which is a current mode type or a voltage mode type.

12. The DC/DC converter according to claim 1 , which is one of a step-down type, a step-up type, or a step-up/down type.

13. The DC/DC converter according to claim 2 , wherein the oscillator further includes an oscillator portion arranged to repeat charging and discharging of a capacitor using the oscillator current so as to generate the rectangular wave signal.

14. The DC/DC converter according to claim 2 , wherein the constant current source includes a constant voltage source arranged to generate a constant voltage and a voltage-current converter portion arranged to convert the constant voltage into the constant current.

15. The DC/DC converter according to claim 2 , wherein the linear current source includes a voltage output type differential amplifier arranged to generate a linear voltage corresponding to a voltage between both terminals of the switching element, and a voltage-current converter portion arranged to convert the linear voltage into the linear current.

16. The DC/DC converter according to claim 2 , wherein the linear current source includes a current output type differential amplifier arranged to generate the linear current corresponding to a voltage between both terminals of the switching element.

17. The DC/DC converter according to claim 2 , wherein the oscillator includes a current mirror circuit, which is used for generating each of the constant current and the linear current, adding them, subtracting them, and setting a current ratio between them.

18. A power supply controller comprising: an oscillator arranged to generate a rectangular wave signal; and a driving circuit arranged to drive a switching output stage of a DC/DC converter in synchronization with the rectangular wave signal, wherein the oscillator is operable to set an oscillation frequency of the rectangular wave signal to a fixed value when an output detection current flowing in the switching output stage is larger than a predetermined value, and the oscillator is operable to decrease the oscillation frequency of the rectangular wave signal from the fixed value more as the output detection current becomes smaller when the output detection current is smaller than the predetermined value, wherein the oscillator includes a constant current source arranged to generate a constant current regardless of the output detection current, and a linear current source arranged to generate a linear current responding to the output detection current, and the linear current is fixed to a first current value when the output detection current is smaller than a first threshold value, and the linear current is variably controlled from the first current value to a second current value responding to the output detection current when the output detection current is larger than the first threshold value and smaller than a second threshold value, and the linear current is fixed to the second current value when the output detection current is larger than the second threshold value.

19. A DC/DC converter comprising: a switching element connected to an input voltage so as to be turned on and off; a driving circuit arranged to perform ON/OFF control of the switching element; an inductor arranged to flow a current controlled by the switching element; a smoothing capacitor connected to the inductor so as to perform a rectifying operation together with the inductor; an oscillator arranged to generate a rectangular wave signal for operating the driving circuit; and an output current detecting portion arranged to detect an output detection current flowing in the switching element or the inductor, wherein the oscillator generates the rectangular wave signal at a fixed oscillation frequency when the output detection current is a predetermined value or larger, whereas the oscillator generates the rectangular wave signal at an oscillation frequency lower than the fixed oscillation frequency and proportional to the output detection current when the output detection current is smaller than the predetermined value, wherein the oscillator includes a constant current source arranged to generate a constant current regardless of the output detection current, and a linear current source arranged to generate a linear current responding to the output detection current, wherein the oscillation frequency of the oscillator is set based on oscillator current corresponding to the sum of the constant current and the linear current or a difference between them, wherein the linear current source includes a voltage output type differential amplifier arranged to generate a linear voltage corresponding to a voltage between both terminals of the switching element, and a voltage-current converter portion arranged to convert the linear voltage into the linear current.

20. The DC/DC converter according to claim 19 , wherein the linear current is fixed to a first current value when the output detection current is smaller than a first threshold value, and the linear current is variably controlled from the first current value to a second current value responding to the output detection current when the output detection current is larger than the first threshold value and smaller than a second threshold value, and the linear current is fixed to the second current value when the output detection current is larger than the second threshold value.

21. The DC/DC converter according to claim 19 , wherein a PWM control is performed when the output detection current is a predetermined value or larger, and a PFM control is performed when the output detection current is smaller than the predetermined value.

22. The DC/DC converter according to claim 21 , wherein setting of an oscillation frequency of the oscillator is controlled by the linear current when the PFM control is performed.

23. The DC/DC converter according to claim 21 , wherein a minimum value of the oscillation frequency of the oscillator is above an audible frequency band when the PFM control is performed.

24. The DC/DC converter according to claim 19 , further comprising: an error amplifier arranged to compare a feedback voltage generated based on a voltage generated at the smoothing capacitor with a reference voltage having a predetermined value so as to output a difference between the both voltages as an error signal; a slope voltage generating circuit arranged to generate a slope signal based on a rectangular wave signal generated by the oscillator; and a PWM comparator arranged to output a signal of a comparison result between the error signal and the slope signal.

25. The DC/DC converter according to claim 24 , wherein a voltage component corresponding to the output detection current is superimposed on the slope signal.

26. The DC/DC converter according to claim 19 , further comprising a rectifier element connected to a common node with the switching element and the inductor so as to supply current to the inductor when the switching element is turned off.

27. The DC/DC converter according to claim 26 , further comprising a reverse current detection circuit arranged to detect a reverse current flowing from a ground potential to the common node, wherein when the reverse current detection circuit detects a predetermined reverse current, operation of the rectifier element or the switching element connected to the ground potential is turned off.

28. The DC/DC converter according to claim 24 , wherein the error amplifier is a transconductance type.

29. The DC/DC converter according to claim 19 , which is a current mode type or a voltage mode type.

30. The DC/DC converter according to claim 19 , which is one of a step-down type, a step-up type, or a step-up/down type.

31. The DC/DC converter according to claim 19 , wherein the oscillator further includes an oscillator portion arranged to repeat charging and discharging of a capacitor using the oscillator current so as to generate the rectangular wave signal.

32. The DC/DC converter according to claim 19 , wherein the constant current source includes a constant voltage source arranged to generate a constant voltage and a voltage-current converter portion arranged to convert the constant voltage into the constant current.

33. The DC/DC converter according to claim 19 , wherein the oscillator includes a current mirror circuit, which is operable to generate each of the constant current and the linear current, adding them, subtracting them, and setting a current ratio between them.

34. A DC/DC converter comprising: a switching element connected to an input voltage so as to be turned on and off; a driving circuit arranged to perform ON/OFF control of the switching element; an inductor arranged to flow a current controlled by the switching element; a smoothing capacitor connected to the inductor so as to perform a rectifying operation together with the inductor; an oscillator arranged to generate a rectangular wave signal for operating the driving circuit; and an output current detecting portion arranged to detect output detection current flowing in the switching element or the inductor, wherein the oscillator generates the rectangular wave signal at a fixed oscillation frequency when the output detection current is a predetermined value or larger, whereas the oscillator generates the rectangular wave signal at an oscillation frequency lower than the fixed oscillation frequency and proportional to the output detection current when the output detection current is smaller than the predetermined value, wherein the oscillator includes a constant current source arranged to generate a constant current regardless of the output detection current, and a linear current source arranged to generate a linear current responding to the output detection current, wherein oscillation frequency of the oscillator is set based on an oscillator current corresponding to the sum of the constant current and the linear current or a difference between them, wherein the linear current source includes a current output type differential amplifier arranged to generate the linear current corresponding to a voltage between both terminals of the switching element.

35. The DC/DC converter according to claim 34 , wherein the linear current is fixed to a first current value when the output detection current is smaller than a first threshold value, and the linear current is variably controlled from the first current value to a second current value responding to the output detection current when the output detection current is larger than the first threshold value and smaller than a second threshold value, and the linear current is fixed to the second current value when the output detection current is larger than the second threshold value.

36. The DC/DC converter according to claim 34 , wherein a PWM control is performed when the output detection current is a predetermined value or larger, and a PFM control is performed when the output detection current is smaller than the predetermined value.

37. The DC/DC converter according to claim 36 , wherein setting of an oscillation frequency of the oscillator is controlled by the linear current when the PFM control is performed.

38. The DC/DC converter according to claim 36 , wherein a minimum value of the oscillation frequency of the oscillator is above an audible frequency band when the PFM control is performed.

39. The DC/DC converter according to claim 34 , further comprising: an error amplifier arranged to compare a feedback voltage generated based on a voltage generated at the smoothing capacitor with a reference voltage having a predetermined value so as to output a difference between the both voltages as an error signal; a slope voltage generating circuit arranged to generate a slope signal based on a rectangular wave signal generated by the oscillator; and a PWM comparator arranged to output a signal of a comparison result between the error signal and the slope signal.

40. The DC/DC converter according to claim 39 , wherein a voltage component corresponding to the output detection current is superimposed on the slope signal.

41. The DC/DC converter according to claim 34 , further comprising a rectifier element connected to a common node with the switching element and the inductor so as to supply current to the inductor when the switching element is turned off.

42. The DC/DC converter according to claim 41 , further comprising a reverse current detection circuit arranged to detect a reverse current flowing from a ground potential to the common node, wherein when the reverse current detection circuit detects a predetermined reverse current, operation of the rectifier element or the switching element connected to the ground potential is turned off.

43. The DC/DC converter according to claim 39 , wherein the error amplifier is a transconductance type.

44. The DC/DC converter according to claim 34 which is a current mode type or a voltage mode type.

45. The DC/DC converter according to claim 34 , which is one of a step-down type, a step-up type, or a step-up/down type.

46. The DC/DC converter according to claim 34 , wherein the oscillator further includes an oscillator portion arranged to repeat charging and discharging of a capacitor using the oscillator current so as to generate the rectangular wave signal.

47. The DC/DC converter according to claim 34 , wherein the constant current source includes a constant voltage source arranged to generate a constant voltage and a voltage-current converter portion arranged to convert the constant voltage into the constant current.

48. The DC/DC converter according to claim 34 , wherein the oscillator includes a current mirror circuit, which is operable to generate each of the constant current and the linear current, adding them, subtracting them, and setting a current ratio between them.